This invention relates to improvements in an AC elevator control system for controlling an elevator car driven by an AC motor.
There have been already known AC elevator control systems for controlling the speed of the induction motor used as an electric motor for driving an elevator car, by supplying to the induction motor an AC power with a variable voltage and a variable frequency inverted from a DC power with a constant DC voltage by a pulse width modulation type inverter. In those AC elevator control systems, mechanical energy is transformed to electrical energy during the descent of the elevator car under heavy loading, the ascent thereof under light loading or the deceleration thereof. That electrical energy is then returned back to the DC side through the inverter. When a DC source used is formed of a battery, the latter is excessively charged so as to evolve a harmful gas or gases. This has resulted in a decrease in the lifetime of the battery. Alternatively, when the DC source is formed of a rectifier and a capacitor connected thereacross, the capacitor has increased in voltage thereacross, resulting in a fear that the component forming the inverter may be dielectrically broken down. Furthermore, if a ratio of a voltage applied across the induction motor to a frequency thereof exceeds a predetermined magnitude, then the induction motor has been saturated in magnetic flux flowing therethrough, resulting in an excessive current flowing through the induction motor.
Thus the AC elevator control systems as described above have necessarily included the equipment for treating the regenerative power returned back to the DC side. For example, with the DC source formed of a rectifier and a capacitor connected thereacross, a separate inverter has been connected across the rectifier and controlled by a return-of-power control device connected across the DC side of the firstmentioned inverter. When a DC voltage across the capacitor exceed a predetermined magnitude upon the occurrence of the regenerative power as described above, the return-of-power control device has been operated to actuate the separate inverter with the result that the DC voltage is returned back to the AC side of the rectifier.
However, the separate inverter and the return-of-power device have been too expensive for an induction motor having an efficiency of operation which does not quite offer a problem because of its relatively small capacity. Also, with a three-phase AC source formed of an emergency three-phase generator, adverse influences might occurs. For example, the generator has an increased three-phase voltage thereacross.
Accordingly, it is an object of the present invention to provide a new and improved AC elevator control system capable of consuming, by the interior of an induction motor, a regenerative power developed thereon during the descent of an associated elevator car under heavy loading, the ascent thereof under light loading or the deceleration thereof resulting in the elimination of the necessity of using equipment for treating the regenerative power.